1. Field of the Invention
The present invention relates to an electric motor control apparatus having at least two electric motors acting as a generator and a motor and causing them to operate in cooperation with each other.
2. Related Background Art
There is an already known method of controlling an output of an electric motor, a so-called PWM control for varying the amount of electricity supplied to the electric motor by means of an ON-OFF control action of a switching element provided in an inverter (for example, refer to Japanese Patent No. 3244845).
Referring to FIG. 13, there is shown an example of a system for controlling an output of an electric motor by means of the PWM control, showing a configuration diagram of a hybrid vehicle having an engine 100, a first electric motor 101 and a second electric motor 102 as driving sources. In this type of hybrid vehicle, the first electric motor 101 and the second electric motor 102 act as a motor and a generator, whereby the engine 100 and the first electric motor 101 drive the front wheels and the second electric motor 102 drives the rear wheels.
Then, an electronic control unit (ECU) 107 performs the PWM control with the ON-OFF control action of a switching element forming a first inverter 105 to transform a drive voltage for the first electric motor 101 and a generated voltage of the first electric motor 101, and performs the PWM control with the ON-OFF control action of a switching element forming a second inverter 106 to transform a drive voltage for the second electric motor 102 and a generated voltage of the second electric motor 102.
In this regard, when the first electric motor 101 is operated as a generator to supply a generated output of the first electric motor 101 and an output power of a battery 103 to the second inverter 106 for the purpose of operating the second electric motor 102 as a motor, the ECU 107 has controlled an output voltage of the first inverter 105 by the PWM control so that the output voltage of the first inverter 105 equals an output voltage of the battery 103. Thereafter, the ECU 107 has controlled the output voltage of the second inverter 106 by the PWM control so that the second inverter 106 outputs a drive voltage corresponding to a target output of the second electric motor 102.
When the PWM control is performed for both of the first inverter 105 and the second inverter 106 in this manner, however, there has been a problem of increasing power loss accompanied by the ON-OFF control action of the switching element in each of the inverters.